IQ and Family Size

Cognitive abilities has been recognized to be a key determinant of educational achievement and scholastic success; Thus an insight into the factors influencing cognitive skills can lead to a better understanding of children’s long run outcomes. But does family size play a decisive role in this? Several researchers have attempted to unravel the mystery but only with limited success.

The data from two national surveys of 56,000 white fathers, Judith Blake of the University of California at Los Angeles found that next to the father's educational level, family size is the most vital predictor of the length of the child’s progress in school, which even outweighs the family's socioeconomic status

A presidential commission proposed that both the downward and upward trends of IQ of a generation are dictated primarily by family size: In general, the smaller the family, the higher the children's IQ. Since families have become smaller, the current upswing in scores "will continue for another 16 to 18 years," reports Robert B. Zajonc of the University of Michigan in Ann Arbor.

Confluence Model

According to the confluence model, "the greater the number of children and the shorter the intervals between successive births, the less mature on the average is the intellectual milieu for each child”. For example, an only child is exposed mainly to his parents' adult environment--the way they interact and deal with their problems -- and to adult language. "In contrast," he notes, "a child in a family of 10, whose oldest sibling is 12, is surrounded by intellectually immature individuals" with less-developed vocabularies. Zajonc suggested that the ideal seemed to be a two-child family with a punctuation of more than two years between children.

Controlling Factors

Given the two types of interventions the instruments represent—one (sex composition) is a planned increase in family size based on parental preferences for variety in the sex composition of their children and the other (twin births) is an unplanned shock to family size resulting in two generally lower-birth weight children with zero spacing-- Estimates using sex composition as an instrument steer clear of any negative effect on family size. However, estimates using twins imply that family size has a negative effect on IQ.

The Admixture Hypothesis

The "Admixture hypothesis" suggested by Page & Grandon account for the apparent nexus between birth order and IQ. This school asserts that factors like parental IQ or socioeconomic status may be responsible for both large families and low IQ, making it appear in cross-sectional studies as though high birth order causes lower IQ. Instead, it is possible that parents with lower IQs tend to have more children. If this were true, then it would be expected that the mean IQ score for any given population would gradually decline. But contrarily, mean IQ scores soar somewhat between 5 and 25 points with each successive generation.

James V. Higgins of Michigan State University in East Lansing concedes that larger families correlate with lower IQs among children. In his analysis of 300 families, Higgins reports that "parents of large families tended to have lower IQs, and concludes that the children, therefore, inherited similar IQ levels. Conversely, he says, "those with higher IQs tended to produce children with higher IQs."

Variations of the above study reflect tangible differences in IQ between large and small families. Small families have more advantages and because of them score higher on IQ tests, from the point of view of sufficient access to and utilization of resources by each member. Their research shows that if you test all children in a small family and all children in a large family within families the IQ would be the same but the smaller family would have a higher IQ.

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